The operation of an internal combustion engine such as, for example, a diesel, gasoline, or natural gas engine, may cause the generation of undesirable emissions. These emissions, which may include particulates and oxides of nitrogen (NOx), are generated when fuel is combusted in a combustion chamber of the engine. An exhaust stroke of the engine piston forces exhaust gas, which may include these emissions, from the engine. If no emission reduction measures are in place, these undesirable emissions will eventually be exhausted to the environment.
Research is currently being directed towards decreasing the amount of undesirable emissions that are exhausted to the environment during the operation of the engine. It is expected that improved engine design and improved control over engine operation may lead to a reduction in the generation of undesirable emissions. Many different approaches such as, for example, exhaust gas recirculation, water injection, fuel injection timing, and fuel formulations, have been found to reduce the amount of emissions generated during the operation of the engine. After treatments such as, for example, traps and catalysts, have been found to effectively remove emissions from an exhaust flow. Unfortunately, the implementation of these emission reduction approaches typically results in a decrease in the overall efficiency of the engine.
Additional efforts are being focused on improving engine efficiency to compensate for the efficiency loss due to the emission reduction systems. One such approach to improving the engine efficiency involves adjusting the actuation timing of the engine valves. For example, the actuation timing of the intake and exhaust valves may be modified to implement a variation on the typical diesel or Otto cycle, such as the Miller cycle. In a “late intake” type Miller cycle, the intake valves of the engine are held open during a portion of the compression stroke of the piston. Selective implementation of a variation on the conventional actuation timing such as the Miller cycle, may lead to an improvement in the overall efficiency of the engine.
The engine valves in an internal combustion engine are typically driven by a cam arrangement that is operatively connected to the crankshaft of the engine. The rotation of the crankshaft results in a corresponding rotation of a cam that drives one or more cam followers. The movement of the cam followers results in the actuation of the engine valves. The shape of the cam governs the timing and duration of the valve actuation. As described in U.S. Pat. No. 6,237,551 to Macor et al., issued on May 29, 2001, a “late intake” Miller cycle may be implemented in such a cam arrangement by modifying the shape of the cam to overlap the actuation of the intake valve with the start of the compression stroke of the piston.
However, while valve actuation timing adjustments may provide efficiency benefits, these actuation timing adjustments may also result in detrimental engine performance under certain operating conditions. For example, a late intake Miller cycle may be inefficient when the engine is starting, operating under cold conditions, or experiencing a transient condition such as a sudden increase in engine load. This detrimental engine performance is caused by a decrease in the mass of air flowing through the engine. Especially under cold ambient conditions, the delayed start of compression may lead to cylinder temperatures insufficient to support good combustion and startability.
Thus, to obtain the greatest gains from implementing a variation on conventional valve actuation timing, the engine requires a variable valve actuation system. As noted above, the shape of the driving cam determines the actuation timing of a valve system driven by a cam arrangement. Because the shape of the cam is fixed, this type of arrangement is inflexible and may only be changed during the operation of the engine through the use of complex mechanical mechanisms.
The engine valve actuation system and method of the present invention solves one or more of the problems set forth above.